US President Barack Obama has piled pressure on Russian President Vladimir Putin to force pro-Russian separatists to stop blocking an international investigation into the shootdown of a passenger jet last week.

In remarks at the White House, Obama said Russian-backed separatists in eastern Ukraine have kept investigators away from the site of the downed Malaysian Airlines plane, at times firing their weapons into the air.

"What are they trying to hide?" he said.

Obama said Putin and Russia have a direct responsibility to compel separatists to cooperate with the investigation and that the burden is now on Moscow to insist that separatists stop tampering with the investigation.